The amplification of the radiative effect of the addition of CO2 and other human-emitted greenhouse gases into the atmosphere requires the addition of water vapor to the atmosphere which is assumed to occur primarily from warmer ocean surface temperatures (thus elevated evaporation). However, this increase of water vapor, at least in recent years is either not occurring or is very muted from the predictions made by the IPCC multi-decadal global model predictions.

There is a new paper (h/t to Doug Gleason the weblog post The Hockey Schtick for alerting us to it) which further documents the muted increase of atmospheric water vapor. The paper is

Over 1/4 billion hourly values of temperature and relative humidity observed at 309 stations located across North America during 1948-2010 were studied. The water vapor pressure was determined and seasonal averages were computed. Data were first examined for inhomogeneities using a statistical test to determine whether the data was fit better to a straight line or a straight line plus an abrupt step which may arise from changes in instruments and/or procedure. Trends were then found for data not having discontinuities. Statistically significant warming trends affecting the Midwestern U.S., Canadian prairies and the western Arctic are evident in winter and to a lesser extent in spring while statistically significant increases in water vapor pressure occur primarily in summer for some stations in the eastern half of the U.S. The temperature (water vapor pressure) trends averaged over all stations were 0.30 (0.07), 0.24 (0.06), 0.13 (0.11), 0.11 (0.07) C/decade (hPa/decade) in the winter, spring, summer and autumn seasons, respectively. The averages of these seasonal trends are 0.20 C/decade and 0.07 hPa/decade which correspond to a specific humidity increase of 0.04 g/kg per decade and a relative humidity reduction of 0.5%/decade.

“An increase in the atmospheric moist content has been generally assumed when the lower-tropospheric temperature (Tcol) increases, with relative humidity holding steady. Rather than using simple linear regression, we propose a more rigorous trend detection method that considers time series memory. The autoregressive moving-average (ARMA) parameters for the time series of Tcol, precipitable water vapor (PWAV), and total precipitable water content (PWAT) from the North American Regional Reanalysis data were first computed. We then applied the Monte Carlo method to replicate the ARMA time series samples to estimate the variances of their Ordinary Least Square trends. Student.s t tests showed that Tcol from 1979 to 2006 increased significantly; however, PWAVand PWAT did not. This suggests that atmospheric temperature and water vapor trends do not follow the conjecture of constant relative humidity over North America. We thus urge further evaluations of Tcol, PWAV, and PWAT trends for the globe.”

More recently, there is an excellent summary of this issue in his post

“……there are a number of other studies which conclude that the multi-decadal global climate models as reported by the IPCC are incorrectly simulating the water cycle which includes the amount of water vapor in the atmosphere. These include, for example,

“….models produce precipitation approximately twice as often as that observed and make rainfall far too lightly. This finding reinforces similar findings from other studies based on surface accumulated rainfall measurements. The implications of this dreary state of model depiction of the real world are discussed.”

“…….extended calculation using coupled runs confirms the earlier inference from the AMIP runs that underestimating the negative feedback from cloud albedo and overestimating the positive feedback from the greenhouse effect of water vapor over the tropical Pacific during ENSO is a prevalent problem of climate models.”

The data being collected by Forrest Mims III provides an excellent resource to continue to monitor the changes in water vapor in the atmosphere over time; see

The new Isaac and van Wijngaarden 2012 paper adds to these studies and is yet another documentation of the inability of the multi-decadal global model predictions of the IPCC to skillfully predict even such a basic component of the climate system as the long term water vapor feedback.